The invention concerns machines for manufacturing containers of thermoplastic material by drawing and blow-molding a preform that has previously been produced by injection molding.
Such machines are used in particular to manufacture containers, for example polyethylene terephtalate (PET) bottles.
A machine of this type uses preforms that are appreciably tubular, closed at one of their axial ends, the other axial end being open and already in the definitive shape of the neck of the final container.
Each preform is heated in a thermal conditioning oven so as to bring the preform to a higher temperature than the glass transition temperature of the thermoplastic material. The conditioned preform is transferred to a blow mold in which a cavity is delimited in the shape of the container to be obtained. The preform is placed in the mold so that its open end extends outside the mold. Thus, a blow-molding device can be brought to the neck of the preform in order to inject air under pressure into the preform. Simultaneously, a drawing rod is inserted axially into the preform until it comes to rest against the closed end at the bottom of the preform. The drawing rod also allows better control of the preform""s axial deformation during the blow-molding of the container.
One of the important parameters for the proper operation of such a blow-molding process is good control of the speed of movement of the drawing rod and the perfect synchronization of this movement with the triggering of the blow-molding.
The invention is more particularly intended to be used in a rotary type drawing/blow-molding machine in which the machine has several blow-molding stations mounted on the periphery of a carrousel that is continuously driven in rotation around its axis. Each blow-molding station has one blow mold, a blow-molding device and a drawing device.
According to one known design, the movement of the drawing rod is controlled by a pneumatic cylinder that furnishes the energy required to move the rod; but the rod""s speed is regulated by a cam roller device that allows the axial position of the drawing rod to be perfectly synchronized based on the angular position of the particular blow-molding station around the axis of rotation of the carrousel.
These systems are entirely satisfactory in terms of operation of the blow-molding process. However, there is a need to greatly reduce the consumption of pressurized fluid of these blow-molding machines. One of the sources of consumption of previously-known machines is comprised of cylinders to control the drawing rod of each blow-molding station.
A purpose of the invention, therefore, is to propose a particularly simple and effective method of controlling the movements of the drawing rod without having to use a pressurized fluid, and without sacrificing the control of the triggering point and of the speed of movement of the rod.
To that end, the invention proposes a machine for manufacturing containers made of thermoplastic material by drawing and blow-molding a preform that has previously been produced by injection, of the type having several blow-molding stations that are mounted on a carrousel driven in continuous rotation around its axis with respect to a frame of the machine, and of the type in which each station is equipped with at least one drawing rod that is slidably controlled along its axis in order to draw the preform during the blow-molding operation, characterized in that the drawing rod is controlled by a magnetic device comprising a fixed magnetic guide placed around the axis of rotation of the carrousel and a slide that is integral with the drawing rod and which carries a magnetic block, and in that the block and the guide cooperate by magnetic coupling so that, depending on the angular position of the carrousel, the guide imposes a specific axial displacement on the slide.
According to other characteristics of the invention:
the magnetic coupling is accomplished with no contact;
the block and the guide cooperate opposite each other along a direction perpendicular to the axial direction of movement of the slide;
the guide is separated in several sections;
the machine has two paired guides that are placed side by side, the slide moving between the two paired guides;
at least one of the magnetic blocks is made in the form of a permanent magnet;
the slide has at least a first permanent magnet (32) that cooperates with a guide to control the slide in the direction of a first axial movement, and at least one second permanent magnet to control the slide in the opposite direction; and
the slide is held in one extreme axial position by a magnetic means integral with the carrousel.